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Gravity Catapult 



Make] Projects 



Gravity Catapult 

Written By: William Gurstelle 



TOOLS: 








PARTS: 


Electric drill (1) 








Panic snap (1) 


A drill press is good for drilling straight 




Find them at tack and saddle shops. 


through steel pipe and fittings. 




a 




feed stores, rural hardware stores, or 


handheld will do. 








online. 


Greased) 






• 


Ferrules (10) 


Hammer (1) 








to fit 1/8" wire rope 


Lockina pliers M) 






• 


Turnbuckles (5) 


Pipe wrench (1) 






• 


Welded ring (1) 


Rotary Tool (1) 






• 


Steel pipefittinqs (1) 


such as a Dremel, for cutting 




rope 




Casters (4) 


Sawd) 






• 


Plywood (1) 


to cut plvwood, such as table 




panel 


• 


Lumber d) 


saw. or handheld circular saw 
Socket set (1) 
Tape measured) 






• 
• 

• 

• 
• 


Deck screws (1) 
Door Hinges (10) 
with mounting screws 
Strap hinges (4) 
with mounting screws 
Screw eyes (9) 
Eye bolts (1) 



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Gravity Catapult 



S- hooks (10) 
Hose clamp (1) 
L-hookd) 
Cordd) 
Wire rope (1) 
Steel piped) 
Bolt (1) 
Duct tape (1) 
Wood screws (1) 
Pipe strap (1) 

for the trigger support bracket 
Chain or roped) 

for the trigger. We used a brass door- 
security chain (and used the bracket it 
came with instead of the pipe strap). 
Bolts d) 
for the casters 
Counterweight (1) 
The counterpoise will carry up to 120lbs: 

s of cement mix work 

great. 

Bungee cords (1) 

PVC piped) 

for a projectile trough 

Lumber (1) 

for a deck cross-brace 



SUMMARY 

I have long been enamored of catapults. Invented around 400 B.C., they were used until 
nearly A.D. 1300. So for 1,700 years, catapults were arguably the largest, most expensive, 
and most powerful machines on the planet. 



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Gravity Catapult 



Today, people build catapults for fun: Scout troops, historical reenactors, fathers and 
daughters, beer-stoked college students. Having built more than a few, I've found it's not 
nearly as easy as it might appear. 

First, for any projectile larger than a golf ball, catapults have to be big, and building big 
things can be a challenge in terms of cost and tools. 

Second, there are incredible stresses at work within the moving parts of a catapult. Unless 
good designs and materials are used, wooden support beams break, rods bend, and joints 
collapse in ways unexpected and sometimes even dangerous. 

Third, they are big. Did I already say that? Well, it bears repeating because once you build 
the thing, you need space to use and store it. I've learned that finding a place to store a 
catapult is a huge pain — very few people are willing to park their cars in the driveway to 
free up garage space for their catapults, no matter how much they love to hurl. 

This gravity-powered catapult is fairly simple to build using a minimum number of tools. Built 
with modern materials instead of medieval timbers, it's small and light enough for one person 
to push around. Best of all, it rolls around on wheels and folds flat (sort of) so it can be 
stored in a fraction of the space needed for traditional catapults. 

While the Folding Catapult is customizable, don't get carried away — 120lbs is about the 
maximum counterweight that can be used. 



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Gravity Catapult 



Step 1 — Build the folding deck. 




Folding Deck Layout Diagram 



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• Using a table saw, or better yet a panel saw, cut out the plywood parts from a 4'x8' sheet 
of 3 /4"-thick plywood, referring to the Cutting Diagram (click to enlarge). 

• If you don't have a table saw, don't despair — most lumberyards will cut the pieces 
for you at little cost when you buy the wood. You could also use a handheld circular 
saw. 

• Use the VA" deck screws to fasten the reinforcing plates to the deck, following the Folding 
Deck Layout Diagram. 

• Using an electric drill, mount the door hinges to the deck with their mounting screws (or %' 
flathead wood screws). 



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Gravity Catapult 



Step 2 — Build and rig the uprights. 



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• Cut 4 pieces of 2x6 lumber 28" long and fasten pairs together using 2 1 /2" deck screws to 
make the uprights. 

• Place the uprights on the outboard deck pieces as shown in the Folding Deck Layout 
Diagram (from Step 1) and attach them to the deck with the 4 strap hinges. 

• Check out the placement of the uprights on the deck — each upright is a different 
distance from the edge. This allows the catapult to be folded compactly for storage. 

• Attach 1" pipe flanges to the uprights as shown using VA" deck screws. 



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Gravity Catapult 



Step 3 




• Now make the guy wires. For this step, you'll need to become familiar with using wire 
rope. But hey, this is a valuable skill that'll come in handy on many different projects over 
a maker's career. 

• Assemble 4 wire slings (clamped segments of wire rope) using the measurements given in 
the Wire Sling Diagram. You can either swage the ends with 8 ferrule fittings, or install 8 
wire rope clamps to do the same job. 

• Install the #4 screw eyes on the ends of the uprights as shown in the Wire Sling Diagram, 
and on the outboard deck pieces as shown in the Folding Deck Layout Diagram. 

• Assemble the rigging as shown in the Wire Sling Diagram using the S-hooks, turnbuckles, 
and wire rope slings. 

• Double-check measurements before cutting, and use locking pliers to grip the wire 
rope while cutting and clamping. 

• Tighten carefully — the cables should be taut but not so taut that they bend the 
screw eyes or damage the deck. 






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Gravity Catapult 



Step 4 — Assemble and attach the rotating arm. 







Throwing Arm Diagram 



• Make the counterpoise. Attach two 1" pipe flanges to the counterpoise deck with 3 A" wood 
screws. Connect two 10" nipples, two 90° elbows, two 4" nipples, a tee fitting, and an 8" 
nipple as shown in the Counterpoise Diagram. Twist all connections together securely. 

• There are 2 separate parts of the rotating arm assembly: the throwing arm and the 
counterpoise. 

• Assemble the throwing arm. Drill a 5/16" hole in the center of the pipe cap. Insert a 1 /4"x2" 
bolt and tighten with a nut and lock washer. Wrap duct tape over the exposed threads to 
smooth them. 

• Next, drill two 5/16" holes in the 6' pipe at the 1 /4"x4" eye bolt locations shown in the 
Throwing Arm Diagram. 

• Drill a 3/8" hole through the center of the cross fitting. Insert the 5/16"x6" eye bolt and 
tighten with lock washers and nuts. 



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Gravity Catapult 



Step 5 




• Attach the throwing arm and counterpoise to the pivot. Connect the throwing arm assembly 
to the flanges on the uprights using two 8" nipples, 2 close nipples, and 2 pipe unions. 
Grease the turning threads. 

• Attach the throwing arm to the cross and assemble the rigging as shown in the Throwing 
Arm Diagram, using Vi" eye bolts, aturnbuckle, S-hooks, and another wire rope sling. 
Tighten the wire sling by rotating the turnbuckle. 

• Attach the counterpoise assembly by screwing it into the remaining opening on the cross 
fitting. 

• Refer to the Throwing Arm Diagram. You should be able to make the assembly fit 
perfectly between the 2 flanges attached to the uprights by adjusting the length of 
the engagement of the unions and the close nipples. However, if there's a problem with the 
fit, you can reposition one of the uprights. 

• To save money, we initially used only 1 union (and a 10" nipple) for the main pivot, 
but this made the throwing arm slower, and harder to remove. Use two 8" nipples 
and 2 unions instead. 





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Gravity Catapult 

Step 6 — Make the trigger. 




• Wrap several turns of the 1/8" cord around the panic snap, tie off, and secure with duct 
tape. 

• Place the loop of the panic snap around your finger and pull down on the cord. The panic 
snap should release easily. If not, reposition the cord and retie. 

• Attach the panic snap to the throwing arm using the hose clamp and welded ring at the 
point shown in the Throwing Arm Diagram. 

• Have a helper hold the throwing arm in "ready to fire" position so you can more 
easily take measurements. 



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Gravity Catapult 



Step 7 



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• Use #10 round- head screws to install the pipe strap as a support bracket on the center 
deck as shown. (Here we used a different bracket that came with our brass door chain.) 

• Measure the distance between the support bracket and the panic snap. Cut the chain or 
rope accordingly and affix one end to the support bracket and the other end to the panic 
snap. 

• Mount a #4 screw eye on the center deck next to the support bracket. Run the 1" cord 
through the eye so that when pulled it releases the panic snap. 



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Gravity Catapult 



Step 8 — Make it mobile. 




* 



• Flip the deck over. Position the casters between the reinforcing plates. Drill holes in the 
deck aligned with the holes in the caster mounting plates. Mount the casters to the deck 
bottom using 5/16" bolts with a washer on each side. 

• If it's easier, mark and drill from the top rather than flipping the deck over and 
drilling from the bottom. 

• Attach the prop board to the bottom of the center deck with the last 2 door hinges. Fold it 
down so it supports the deck when the catapult is in use. This prevents rolling during setup 
and firing. 

• Mount the L-hook next to the prop board as shown in the Folding Deck Layout Diagram. 
Rotate the hook to hold the prop board up when moving the catapult. 

• (Optional) To make the deck even stronger, bolt a 2x4 cross-brace on top of the center 
deck with a V6" bolt. Rotate it crosswise during catapult use and lengthwise for storage. 

• Stepping on the deck can damage the strap hinges. To prevent this, bolt an optional 
2x4 cross-brace to the center deck. 






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Gravity Catapult 



Step 9 — Make it accurate (optional). 




Projectiles fly farther and straighter when guided by a smooth channel on the deck. To 
make a projectile trough, saw a 4' length of 4" PVC pipe in half lengthwise and use deck 
screws to attach it to the deck directly below the throwing arm. 



Safety Notes 

Keep all body parts away from the throwing arm's swing area at all times. Warn downrange bystanders of possible incoming 
projectiles. Use care when pushing the catapult. Don't step or stand on the deck. Remember that all hinges are pinch hazards. 
Keep hands and feet clear when folding. 

Ready... 

To move the catapult, fold the prop board up and twist the L-hook to keep it in place. Push the 
catapult to a location with at least 150' of open space to hurl, with no nearby vulnerable targets, 
such as people, pets, or vehicles. Place the prop board in the down position. 

Place up to 120lbs of weight on the counterpoise deck. (Two 60lb bags of concrete mix fit quite 
well.) If desired, attach two 18" bungee cords to the deck and counterpoise. This will increase 
the machine's range and capacity. 

WARNING: Don't trust the panic snap to hold the arm. Have a helper hold the arm while you 
latch it, and keep your head and other body parts out of the arm's arc at all times. 



Point the catapult at the target. Pull the arm down. Latch the panic snap to the firing ring. 

Attach a rope or cord to a soft, lightweight test item (plastic dog toys are a good choice). 

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Gravity Catapult 

Determining the optimum length of the sling is done through trial and error. Start with a sling 
length 1/3 the length of the throwing arm and work from there. Be aware that too short a sling will 
hurl the projectile backward. 

Tie a nontightening loop in the free end of the sling (a bowline is perfect) and place the loop over 
the W bolt protruding from the pipe cap. Center the projectile on the center deck. 



Make sure the area in front and in back of the catapult is clear. Step 8 or 9 paces to the side. 
Grasp the cord and pull smartly to release the panic snap. The throwing arm will rotate and hurl 
the projectile toward the target. 

You can optimize performance by making the sling longer or shorter, making the firing pin 
smoother or rougher, and adding or removing weight from the counterpoise. 



The Folding Catapult is designed for easy storage. First, remove the weight from the 
counterpoise deck. 

Loosen the turnbuckle on the throwing arm, then loosen and remove the pipe unions on the 
throwing arm pivot. Remove the throwing arm. Finally, unscrew the pivot assembly pipe stubs 
from the flanges on the uprights. 

Unscrew and remove the counterpoise assembly from the cross fitting. 

Loosen turnbuckles and remove the guy wires from the screw eyes, making sure to collect all 
loose S-hooks. Fold the uprights, and then fold the deck for storage. 

This project first appeared in MAKE Volume 28 . page 84. 



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